In 1999 one of the scores of death-row appeals that land at the Supreme Court each year caught the eye of Justice Stephen Breyer. Thomas Knight, who had been sentenced to death by the State of Florida in 1975 for killing a married couple, argued in the appeal that he had been living in anguished anticipation of execution for so long that his sentence had become a form of cruel and unusual punishment. Knight's case was joined with that of Carey Moore, who'd been on Nebraska's death row for nineteen years for killing two cabdrivers.

No court in the United States had ever lifted a death sentence for this reason. But Breyer had a hunch that courts in other countries might have done so, and he asked his clerks to investigate. He had issued a similar directive earlier that term, when he wanted to know more about other countries' approaches to campaign-finance law. "I remember his exact words," says one of Breyer's clerks from that year. "He said, 'We're not the only court in the world. See what they have to say.'" Breyer has come to refer to proponents of this approach—namely, judges who use international legal precedents for context as they interpret the U.S. Constitution—as "comparativists."

In their death-penalty research Breyer's clerks found several cases in which courts from around the world, including the European Court of Human Rights and the Supreme Court of India, had expressed serious reservations about whether executions conducted after lengthy delays were humane. When the Court declined to review Knight and Moore's case, Breyer took the unusual step of commenting on it anyway. In a dissent to the Court's denial of review he wrote, "A growing number of courts outside the United States—courts that accept or assume the lawfulness of the death penalty—have held that lengthy delay in administering a lawful death penalty renders ultimate execution inhuman, degrading, or unusually cruel."

None of Breyer's colleagues signed on to the dissent. And in a preview of the resounding rejection of comparativism soon to come from other hard-line conservatives, Clarence Thomas wrote an opinion ridiculing Breyer's approach. "Were there any support in our own jurisprudence," he argued, "it would be unnecessary for proponents of the claim to rely on the European Court of Human Rights, the Supreme Court of Zimbabwe, the Supreme Court of India, or the Privy Council."

Twenty years ago the exchange of law among nations consisted almost entirely of a "vigorous overseas trade in the Bill of Rights," as one British barrister has put it. Canada, for example, followed the United States' lead on freedom of religion; India did so in balancing the values of freedom of expression and privacy in libel law. But times have changed, and many countries' highest courts have now come into their own. The courts of South Africa and India, for example, have upheld rights to education, housing, and health or medical care that American courts have not recognized. In Europe, too, courts are becoming bolder, despite a tradition of parliamentary supremacy. Breyer feels strongly that the United States can learn from these developments abroad. "Really, it isn't true that England is the moon, nor is India," he said in a debate with Justice Antonin Scalia last January at American University, in Washington, D.C. Judges in those countries "have problems … similar to our own."

Breyer is no longer a lonely crusader: comparativism is startlingly in the ascendant in the U.S. Supreme Court, among both the relatively liberal justices and the centrist conservatives. Foreign precedents were at the heart of the Court's recent decisions to strike down sodomy laws (Lawrence v. Texas, 2003) and to ban the execution of juveniles (Roper v. Simmons, 2005). Both opinions were written by Justice Anthony Kennedy, who often votes with the Court's conservative bloc. They did not sit well with observers on the right. Some conservatives have called for Kennedy's impeachment—and have introduced bills in the House and Senate that would ban American courts from employing any foreign judgment other than centuries-old English common law when interpreting the Constitution.

Kennedy and the other justices who have sympathized with comparativism (Breyer, Ruth Bader Ginsburg, David Souter, and John Paul Stevens, along with Sandra Day O'Connor, who announced her resignation last summer) have trod lightly in view of the conservative uproar. Breyer argues that comparativism has a modest aim—to "cast an empirical light" that will reveal new solutions to shared legal problems. He and his allies have pointed out that looking abroad for precedents is not new for the Court: in cases from the 1950s through the 1980s (and also during the 1890s) majorities took account of foreign practices in deciding whether a punishment was permissible. This attracted little notice until Scalia decreed, in the 1989 case Stanford v. Kentucky (which allowed juvenile execution), that in weighing whether a punishment was cruel and unusual the Court would consider only "American conceptions of decency" (his italics).

In overturning Stanford this year the Roper majority stressed that American courts are not bound to follow rulings from abroad. But of course foreign judgments don't need to be formally binding to be persuasive. Kennedy observed in his opinion that the United States was the only country in the world that still executed minors.